The sheet delivery mechanisms in printing machines typically possess front and rear sheet stops for the alignment of the sheets to be deposited and a push-in auxiliary-stack board or rake for intercepting sheets during the stack change. DE 2,301,840 A1 discloses a sheet-delivery apparatus, in which, in the region of the front edge of the sheet to be deposited, bearing fingers can be pivoted inwards over the main stack. In a further design, for the purpose of the stack change, an auxiliary-stack table can be pushed in over the main stack and sheet retention devices, which are arranged on the side located opposite the bearing fingers, can be introduced above the stack by means of working cylinders.
An apparatus for the intermediate interception of sheets, particularly for holding the sheet rear edge, is know from DE 3,112,558 C2. As disclosed there, angle supports are arranged rotatably on a holder in the region of the sheet brake and serve as supporting means for the sheet rear edge. The angle supports have different leg lengths and can swing away as a result of the weight of the sheets, when a plurality of sheets are laid on them, so that the part stack obtained passes onto the intermediate-stack board, which in the meantime has been pushed in.
DE 3,112,672 C1 discloses a stop for the sheet rear edge, which is designed as a crossmember and which, together with supporting shears fastened to the sheet brake, is arranged so as to be movable up and down in conjunction with an intermediate-stack device.
DE 3,937,945 C2 discloses a sheet rear-edge catching device for deliveries having a non-stop auxiliary-stack device. Controllable catching fingers are arranged underneath the suction roller between rear-edge stops and are movable jointly in one plane by a pneumatic working cylinder operating a rack and pinion.
In the abovementioned references DE 2,301,840 A1 and DE 3,112,558 C2, it is impossible to prevent oncoming sheets from slipping back into the machine in the direction of the sheet feed, or from being jammed or deformed between the rear-edge stop and auxiliary stack.
According to DE 3,112,672 C1, the oncoming sheets are prevented from slipping back, but the stop cannot achieve the object of an exact separation of the sheets of the main and auxiliary stacks.
According to DE 3,937,945 C2, the oncoming sheets are prevented from slipping back, but, a complicated design for the separation of the main and auxiliary stacks is necessary. Actions in which sheets are speared in the region of the rear edge are avoided during the pushing in of the rake, but the risk is merely transferred to the horizontal reciprocating movement of the catching fingers. Particularly at higher machine speeds (for example, four sheets per second), spearing actions by the catching fingers are unavoidable.
As a further development of DE 3,937,945 C2, a device, is described in DE 4,011,286 A1 wherein the catching fingers are designed as suction grippers. On the top side of the suction grippers, the deposited sheets are sucked up by means of suction nozzles, and in order to tension the sheets the suction grippers are retracted by a specific amount. The suction grippers move horizontally, in a similar way to DE 3,937,945 C2, and do not overcome the disadvantages already indicated.